The expansion of the utilization of renewable energies, which has accelerated in recent years, in particular the construction of numerous photovoltaic and wind power plants, the energy production of which is dependent on uncontrollable environmental conditions, has led to the realization that, in order to ensure a continuous supply of energy in accordance with demand, energy stores are of major significance. Accordingly, the development of energy stores, in particular energy stores which have a large storage capacity, have taken on great significance.
One category of such energy stores are positional energy stores, in the case of which excess energy is utilized for increasing the potential energy of a (large) mass. A category of positional energy stores that has been known for many years is that of pumped-storage power plants, in which water is pumped into a reservoir at a greater height in order to store potential energy. The capacity of individual pumped-storage power plants is however limited, and the number thereof cannot be increased arbitrarily owing to great requirements placed on the site.
Said disadvantages are avoided in the case of a new category of positional energy stores. In the case of said positional energy stores, an example of which is known from DE 10 2010 034 757 B4 and which will hereinafter be referred to as “generic positional energy stores”, a large mass is raised relative to the earth's surface using a hydraulic fluid, for example water, in a hydraulic cylinder by virtue of the hydraulic fluid being pumped in via one or more lines, such that the mass practically constitutes the piston which is moved in the hydraulic cylinder, and energy is stored as potential energy of the raised piston.
When the piston has been raised as far as possible in the hydraulic cylinder, or when no excess energy is presently available for storage, the lines are closed off by way of valves or locks, and the piston remains in its present position.
The potential energy of the raised piston can then be converted into electricity again when the valves or locks are opened, without the pumps being operated. Then, hydraulic fluid is displaced out of the hydraulic cylinder, and the flow energy thereof can be converted into electrical energy again in a known manner by way of generators, similarly to the situation in conventional pumped-storage power plants.
Here, the mass may be formed in particular by a cut-out rock, and the required hydraulic cylinder may be formed by the stone surrounding the cut-out rock.
It is self-evidently necessary in the case of such positional energy stores for a seal to be arranged between the large mass, for example the rock mass, and the hydraulic cylinder, for example the surrounding stone, in order to prevent the uncontrolled escape of the hydraulic fluid, bypassing the line system.
The major advantage of said construction, in the case of which it is possible to realize piston diameters and lifting heights of several hundred meters, is the very high storage capacity of such installations, which greatly exceeds that of conventional storage power plants.
A positional energy store of the generic type described above is disclosed for example by DE 10 2010 034 757 B4.
Development work has shown that the provision of a well-suited seal ring is difficult, and the direct transfer of known sealing concepts leads to sub-optimal results.
It is a problem here that, in the case of high pressures to be managed, it is firstly the case that a relatively wide gap is present in relation to customary gap sizes to be sealed off, because the spacing between the working mass, which is formed by the piston, and the surrounding stone, which forms the wall of the hydraulic cylinder, has a typical minimum extent of several hundred millimeters up to 1000 mm.
Even if a relatively small extent of the spacing could possibly be technically realized in the uncharged state, this would not be desirable because, then, the rock mass, which forms the piston, is, in an at least partially deployed state, subject to a not inconsiderable wind pressure, which gives rise to slight tilting of the piston-forming mass relative to the hydraulic cylinder. In the case of an excessively small spacing between the mass and the hydraulic cylinder, jamming of the mass in the hydraulic cylinder could occur in said situation.
Specifically the tilting of the piston-forming mass in the hydraulic cylinder however intensifies the sealing problem yet further in that then, it is firstly the case that the width of the gap to be sealed off can vary along the lifting direction, and secondly, said width may also differ in different directions perpendicular to the lifting direction.
Furthermore, as a further peculiarity of the gap to be sealed off between piston and hydraulic cylinder, it is the case that the gap width varies in a manner dependent on the state of charge thereof, which correlates with the present lifting height of the piston. This can be attributed to the fact that the pressure of the hydraulic fluid that acts on the piston and hydraulic cylinder is dependent on said variable, and is of such a magnitude that the materials of which the components are composed are compressed. These effects lie in the range of decimeters in the case of positional energy stores with radii of a few hundred meters.
As a further problem, it is also the case that the surfaces of the piston and/or hydraulic cylinder do not define a perfect prismoidal body and are not sustained but are uneven and thus have a roughness not only on a microscopic scale. For example, mining work required for cutting a rock mass, which forms the piston of the hydraulic cylinder, out of the surrounding stone gives rise to structured stone surfaces. These rough surfaces increase demands on the seal in two respects. Firstly, the seal must be flexible enough that, when it comes into contact with a structured surface, it prevents the passage of fluid at every point of the surface structure. Secondly, however, it must be ensured that the seal is not subject to excessive wear in the event of a change in the position of the rock mass, which forms the piston of the lifting cylinder, in the lifting cylinder, such as is essential for the operation of the positional energy store.
It is therefore desirable to provide a positional energy store with an improved seal.